fix(makeXmlContTypes): correct jpg MIME + drop ghost slideMaster Overrides (closes #1444)

[IgnasStark]

Summary

Two long-standing bugs in [Content_Types].xml generation, both inside makeXmlContTypes() at src/gen-xml.ts. Both fixed in one PR because they share the same two-line region of the function and are mechanically tiny.

Bug 1 — image/jpg is not an IANA-registered MIME

-	strXml += '<Default Extension="jpg" ContentType="image/jpg"/>'
+	strXml += '<Default Extension="jpg" ContentType="image/jpeg"/>'

image/jpg does not exist in the IANA registry — the valid type is image/jpeg. PPT files generated with image/jpg open in lenient parsers (PowerPoint Desktop, Keynote, LibreOffice, Google Slides) but trigger strict-mode warnings in OOXML linters and some compliance-focused parsers (e.g. SoftMaker FreeOffice). The file extension on the binary stays .jpg; only the MIME changes.

Bug 2 — ghost slideMaster Overrides (closes #1444)

 strXml += '<Override PartName="/ppt/notesMasters/notesMaster1.xml" ContentType="application/vnd.openxmlformats-officedocument.presentationml.notesMaster+xml"/>'
+strXml += '<Override PartName="/ppt/slideMasters/slideMaster1.xml" ContentType="application/vnd.openxmlformats-officedocument.presentationml.slideMaster+xml"/>'
 slides.forEach((slide, idx) => {
-    strXml += `<Override PartName="/ppt/slideMasters/slideMaster${idx + 1}.xml" ContentType="application/vnd.openxmlformats-officedocument.presentationml.slideMaster+xml"/>`
     strXml += `<Override PartName="/ppt/slides/slide${idx + 1}.xml" ContentType="application/vnd.openxmlformats-officedocument.presentationml.slide+xml"/>`

The slides.forEach loop currently emits one <Override PartName="/ppt/slideMasters/slideMaster${idx + 1}.xml"/> per slide, but PptxGenJS only ever serializes a single slideMaster1.xml to the zip (see genMasterSlide() which writes one master regardless of slide count). Result: an N-slide deck declares N slideMaster Overrides in [Content_Types].xml but only slideMaster1.xml exists on disk — N-1 ghost references.

PowerPoint Desktop tolerates this and silently ignores the ghosts. Stricter parsers report orphan Overrides; some online viewers (and PowerPoint Repair if the file is round-tripped) flag the file as corrupt and rewrite it.

The fix is structural: hoist a single literal slideMaster1.xml Override above the slides.forEach and drop the per-slide templated line. The forEach still iterates for slides/slide${idx+1}.xml Overrides and chart Overrides — only the slideMaster line is hoisted out.

Tracked at issue #1444 (open Feb 2026).

Verification

Generated a 2-slide deck with this branch applied, inspected [Content_Types].xml:

metric	before	after
slideMaster<N>.xml Override count for 2-slide deck	2 (ghost)	1
Extension="jpg" ContentType="image/jpg"	1	0
Extension="jpg" ContentType="image/jpeg"	0	1
slide<N>.xml Override count	2	2 (unchanged)
File opens in PowerPoint Desktop / Keynote / Slides / LibreOffice	✓	✓

No behaviour change for lenient parsers; strict-mode warnings cleared.

Context

Downstream consumer (Skaylink Presentation Generator) is shipping this exact patch via patch-package against pptxgenjs@4.0.1 because the upstream issue has been open since Feb 2026 without movement. Filing this PR as the courtesy upstream contribution so future versions don't need the patch.